1. Technical Field
The invention disclosed broadly relates to improvements in digital computer architecture and more particularly relates to an improved stylus detection subsystem used in conjunction with a graphic input grid overlay which is positioned on the viewing surface of a display device.
2. Background Art
Modern microcomputer systems generally include a central processing unit (CPU) which is connected by means of an address bus and a data bus to a plurality of input/output (I/O) devices such as a keyboard, a display device, serial communications devices, printers, and other I/O equipment. It is well recognized that graphical representations are a powerful and efficient means for characterizing data, the graphical representations being displayed on a display device such as a raster scanned cathode ray tube display, a gas panel display, a light emitting diode array display, and the like. The power of graphics as a medium to improve the man-machine communication is usually limited, however, because of the uni-directional flow of the information, since typical data processing systems can present data in the form of graphical information to the user, however the user is limited in his ability to communicate with the system by inputting graphical information.
The prior art has attempted to provide graphical input devices in the form of graphic input tablets. Such tallets are opaque platforms containing a grid work of horizontal and vertical wires which sequentially carry a radiative signal which is picked up by a stylus brought into close proximity to the tablet. The coordinates for the relative position of the stylus with respect to the tablet are correlated with the sequential radiation from the respective wires in the tablet, thereby producing a digital value for the stylus position which can be input to the system. The problem with such opaque graphic input tablets is that it is difficult for the user to position the stylus on the tablet so that corresponding tablet position and stylus position images displayed on the display device can be adjusted as desired by the user.
The prior art has attempted to improve the correlation between the relative position of the stylus and the radiative wires and the corresponding displayed positions on the display device, by providing a transparent, graphic input grid overlay which is positioned on the viewing surface of the display device. In a manner similar to that used in prior art opaque graphic tablets, the graphic input grid overlay has an array of horizontal and vertical wires which are sequentially energized with a signal so that they sequentially radiate electromagnetic radiation. That electromagnetic radiation can be picked up by a stylus and thus coordinates are established for the relative position of the stylus with respect to the wire grid. The system can then display on the display device the corresponding relative position of the stylus and other images depicted on the display. The problem with this prior art approach is the parallax effect which the viewer suffers in attempting to locate the stylus at a point over the viewing surface of the display. The parallax occurs because the user is focusing on the physical location of the stylus, and is attempting to bring that physical location into close contact with a preexisting image displayed on the display device. Since the user generally does not have his eye exactly aligned with the stylus and further, since the graphic input grid overlay may not have its constituent wire grid precisely registered with respect to the position of that grid which is attributed by the display device, there is usually a parallax error in the positioning of the stylus.